The adsorption and dissociation of carbon monoxide on the W(111) surface is studied with density functional theory. The CO molecule is found to adsorb in end-on configurations (alpha states) and inclined configurations (beta states). The dissociation of the most strongly bound beta state CO is found to have an activation energy of about 0.8 eV, which is lower than the energy required to desorb CO molecularly from the surface. The diffusion of CO and O on W(111) is predicted to be facile at room temperature, whereas C atoms are virtually immobile up to similar to 600 K, according to our calculations. Preadsorbed carbon atoms are shown to prevent the dissociation of CO by blocking the most strongly bound P state adsorption site and by blocking the dissociation pathway. We predict that dissociation of CO on W(111) is a self-poisoning process.